One area of manufacturing research that has made significant technological advancements in recent years is high-speed machining. Machine improvements include new spindle designs for higher rotational speed, torque, and power; increased slide speeds, accelerations, and direct drive linear motor technology; and new machine designs for lower moving mass. The combination of new machine technology and tool material/coating developments makes high-speed machining a viable alternative to other manufacturing processes. For example, high-speed machining has been applied in the aerospace industry, where the dramatic increases in material removal rates made possible using high-speed machining techniques have allowed designers to replace assembly-intensive sheet metal build-ups with monolithic aluminum components, resulting in substantial cost savings.
However, during a metal cutting operation, any vibratory motion between a cutting tool and workpiece may lead to non-beneficial cutting performances. Furthermore, such vibration may cause the cutting tool or the machine tool to become damaged. Excessive vibrations, frequently called “chatter,” between the cutting element of a machine tool and the surface of the workpiece cause poor surface finish, tool breakage and other unwanted effects that have long plagued machining operations. Such vibrations arise especially when the tool includes a long unsupported length that will permit deflection of the tool. When chatter does occur the machining parameters should be changed and as a result productivity may be adversely affected. In machining operations, chatter is also one of the primary limitations to high material removal rates.
Many research efforts geared toward the understanding and avoidance of chatter have been carried out. This work has led to the development of stability lobe diagrams that identify stable and unstable cutting zones as a function of the chip width and spindle speed. However, the methods used to produce these diagrams, whether analytic or time-domain, require knowledge of the tool point dynamics. The required dynamic model is typically obtained using impact testing, where an instrumented hammer is used to excite the tool at its free end (i.e., the tool point) and the resulting vibration measured using an appropriate transducer, typically a low mass accelerometer. However, due to the large number of spindle, holder, and tool combinations, the required testing time may be significant.
One research effort, in particular, is set forth in U.S. Pat. No. 5,170,358 to Delio. In this reference, the cutting signal obtained while machining is monitored and the spectral content is observed to identify preferred spindle speeds for chatter avoidance. However, this reference requires that the measurements be performed after cutting tests. The method makes no attempt to produce a pre-process stability lobe diagram. Rather, by using information obtained during an actual chattering (unstable) cut at a selected axial depth of cut, recommendations for chatter free spindle speed(s) may be made.
To produce a stability lobe diagram, the dynamic response of the machine-spindle-holder-tool (as reflected at the free end of the tool, or tool point) should be known. This data may be obtained by performing a dynamic test on the machine-spindle-holder-tool in question, but the dynamic response changes if a new holder or tool (or new tool length) is used, so the measurement should be repeated. This requirement is often not practical for a production environment.
Accordingly, it would be beneficial to provide a model that is able to predict a tool point response based on minimum input. It would also be beneficial to provide a model that is able to predict a tool point response to permit a stability lobe diagram to be created for a particular spindle-holder-tool.